1. Field of the Invention
The present invention relates to a field of simultaneous transmission and reception of a signal in a communication system having a transmitter and a receiver such as a portable telephone. In particular, the invention relates to a radio communication apparatus in the communication system using a modulation method having an envelope component such as a CDMA method.
2. Related Art of the Invention
In recent years, a cellular radio communication systems have been rapidly increasing due to the pressing needs for mobile communication and development of communication technology.
An example of a radio communication apparatus used in the cellular radio communication system is shown in a block diagram in FIG. 31. In this drawing, reference numeral 1801 denotes an antenna, 1802 denotes a duplexer (antenna sharing apparatus), 1803 denotes a receiving circuit, and 1804 denotes a transmitting circuit.
In the radio communication apparatus shown in FIG. 31, a radio frequency signal transmitted from a base station is received by the antenna 1801 and is then inputted via the duplexer 1802 to the receiving circuit 1803. In receiving circuit 1803 the signal is high-frequency-amplified unnecessary waves outside a receiving band are eliminated and the signal is then converted into an intermediate-frequency signal. The intermediate-frequency signal is demodulated and converted into a base band signal. Predetermined signal processing is performed to a transmitting base band signal which is then inputted to a transmitting circuit 1804. A carrier wave signal is modulated so that the modulated carrier wave signal is converted into a radio frequency and amplified to a predetermined sending power to be sent via the duplexer 1802 from the antenna 1801 to the base station.
The receiving circuit 1803 uses a low-noise amplifier as a high-frequency amplifier. In the case where the transmitting signal leak is not completely attenuated by the duplexer it is inputted to the low-noise amplifier. The following three factors cause deterioration of reception.
Firstly, it is because of deterioration of a noise characteristic of the low-noise amplifier itself due to the transmitting signal leak of a large amount of power. FIG. 32 shows an example of a relationship between a transmitting signal leak level and a noise figure. It is caused because current noise increases due to increase in currents of the low-noise amplifier caused by a transmitting signal leak, or high-frequency thermal noise is down-converted in the receiving band or inversely low-frequency thermal noise is up-converted in the receiving band due to the transmitting signal leak.
Secondly, it is because of gain compression of the low-noise amplifier due to the transmitting signal leak of the large amount of power. FIG. 33 shows a relationship between the transmitting signal leak level and the gain. Normally, the noise figure of the circuit in the subsequent stage to the low-noise amplifier is lower than the noise figure of the low-noise amplifier by 5 to 10 dB. If the gain of the low-noise amplifier is sufficiently high, it is possible to reduce influence of the noise characteristic of the circuit in the subsequent stage. If the gain of the low-noise amplifier is reduced by the transmitting signal leak, however, the influence of the noise characteristic of the circuit in the subsequent stage becomes significant so that the reception is consequently deteriorated.
Thirdly, it is because of intermodulation due to the transmitting signal leak of the large amount of power. In the case of a portable telephone or the like adopting a CDMA method, its own transmitting wave has an amplitude variable component as shown in FIG. 34 for instance. For this reason, if there is a narrow-band jammer of an analog cellular system using a near band of a CDMA cellular system in the proximity of a wave desired to be received for instance, the amplitude variable component of the transmitting signal leak causes the intermodulation due to third order distortion of the low-noise amplifier. And as shown in FIG. 35, it moves to the jammer, and a part of it is added to the receiving band as interference.
The CDMA cellular system adopts so-called open-loop sending power control of increasing transmitting signal power when the radio communication apparatus is distant from the base station and a receiving signal level is low. In the case where the radio communication apparatus exists in a fringe area of a cell for instance, reception performance deteriorates at an accelerated pace due to the interference by the intermodulation action. In the worst case, there is a possibility that a call may be interrupted.
To avoid these problems, it is necessary to set an input 1 dB gain compression point (P1dB) of the low-noise amplifier provided on a receiving high-frequency unit at −5 to −3 dBm or so, and set an input third order intercept point (IIP3) thereof at +5 to 7 dBm or so. To realize this, however, it is necessary to increase current consumption to over 10 to over 20 mA. And there is a problem that, if done so, standby time of the CDMA radio communication apparatus is significantly reduced, which is not desirable.
“Radio Transceiver and its Receiving High-Frequency Unit and Control Unit” (refer to Japanese Patent Laid-Open No. 11-274968) and “Mobile Communication Apparatus” (refer to Japanese Patent Laid-Open No. 2000-286746) are known as past examples of the radio communication apparatus of improving the reduction of the standby time caused by deteriorated reception due to its own transmitting signal leak not completely attenuated by the duplexer. Either method comprises a mode of increasing the current consumption and implementing low distortion and a mode of implementing low current consumption, and the modes are switched such as the low distortion mode on simultaneous transmission and reception and the low current consumption mode during non-transmission time so as to improve standby time performance. As for the method of implementing the mode switching, Japanese Patent Laid-Open No. 11-274968 describes use of the method of switching two types of low-noise amplifiers with a high-frequency switch, and Japanese Patent Laid-Open No. 2000-286746 describes use of the method of switching the currents running in the low-noise amplifier.
A down mixer is used for the receiving circuit 1803 for the sake of frequency conversion. As for this down mixer, the reception deteriorates due to the transmitting signal leak as with the low-noise amplifier. If the low-noise amplifier is directly connected to the down mixer, the transmitting signal leak amplified by the low-noise amplifier is inputted to the down mixer so that the reception further deteriorates. If the current of the down mixer is increased in order to improve it, the current consumption more than that of the low-noise amplifier is required. For that reason, an inter-stage filter is normally used between the low-noise amplifier and the down mixer so as to reduce the transmitting signal leak inputted to the down mixer. The disclosures of the above-mentioned documents are incorporated herein by references in their entireties.